Enhancing the efficiency of low bandgap conducting polymer bulk heterojunction solar cells using P3HT as a morphology control agent

The development of low bandgap conducting polymers has made bulk heterojunction solar cells a viable low cost renewable energy source. The high boiling point of 1,8-diiodooctane (DIO) is usually used to control the morphology of the active layer consisting of a conducting polymer and PCBM, so that a high power conversion solar cell can be achieved. We report here an alternative approach using nonvolatile, crystalline and conducting P3HT as an effective morphology control agent. A model system of PCPDTBT/PC61BM was selected for this study. The change of optoelectronic properties with the introduction of P3HT was monitored by measuring the absorption spectra and charge carrier mobility, and the morphology change with the introduction of P3HT in the active layer was monitored by AFM, TEM, and GIXRD. The results indicate that favorable bi-continuous phase separation and appropriate domain size of each phase can be achieved to facilitate fast charge transport, and thus improve the power conversion efficiency of the solar cell. By adding 1 wt% P3HT into the blend of PCPDTBT/PC61BM, the power conversion efficiency can be improved by 20%. Moreover, with the incorporation of 1 wt% P3HT to the blend of PCPDTBT/PC61BM with DIO, the power conversion efficiency can be further increased by 17%. The strategy of this study can be expanded to other low bandgap conducting polymers for high efficiency bulk heterojunction solar cells.